The present invention relates generally to the field of electric motor repair and more specifically to the repair of a commutator to brush interface in a direct current motor without dismantling the motor.
In particular operations using a direct current, the commutator to brush interface can get dirty or covered with a coating commonly called a "varnish" which can cause intermittent and unreliable operation. The speed can vary causing undependable rotation. In a tape drive for a data processing system, constant motion of a tape over a transducer is compulsory. Any hindrance to a constant speed causes errors in the reading or writing of the data from the tape which can result in a failure in the data processing system. The rotational speed of the motor and thereby the linear speed of the tape must not vary. Reel to reel motion control systems in tape drives set the DC tension on the tape using calculated motor currents. When the motors are run at low currents for long periods of time the brushes get dirty and the connection to the commutator becomes unreliable. This results in data errors and tape damage when the system tries to move the tape and maintain the proper tension of the tape between the two reels. Brush motors cannot operate with small currents through the brushes for long periods of time in a servo control application such as magnetic tape drives. A high current density through the brush-commutator interface as well as high speeds are required to keep the interface clean and thereby maintain speed reliability. These are not always available especially in tape drives used in data processing systems. The tape has gotten very thin and consequently the tape drive required much lower current to maintain tape tension between the reels. the currents required for steady state motion are low once the tape is moving since the energy required to maintain constant motion is small. For a majority of the time, the motors are operating in a region, low current and low speed, where the self-cleaning action of the brush-commutator interface cannot take place.
A common method for cleaning the commutator-brush interface of a motor to spray the commutator with FREON, a chlorofluorocarbon compound, and then let the compound dry. An advantage of this cleaning method is that the FREON evaporates rapidly thereby eliminating any doubt about the armature windings having dried after being soaked by this cleaning process. Operation of the motor with wet windings must be avoided, as short-circuiting of the windings may occur. The indeterminate drying process and the environmental-unfriendliness attributed to the use of chlorofluorocarbons makes this process unusable, especially in a data processing system.
Another common solution is to dismantle the tape drive and remove and disassemble the motor for cleaning. In a small data processing system, the disconnection of the tape drive unit completely disrupts the operation of the data processing system and essentially shuts down its function. In a large data process unit, at minimum the productivity is affected.
An object of the present invention, therefore, is to provide a process for reinstating the reliability of a motor, especially a motor used in a tape drive, without dismantling the tape drive and/or the motor and without the use of unfriendly, environmentally undesirable products.